While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device includes an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, and referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element including a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of a light-emitting dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant that has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases, singlet excitons can also transfer their energy to the lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
Fac-Tris(2-Phenylpyridinato-N,C2′)Iridium(III) (Ir(ppy)3) strongly emits green light from a triplet excited state owing to the large spin-orbit coupling of the heavy atom and to the lowest excited state, which is a charge transfer state having a Laporte allowed (orbital symmetry) transition to the ground state (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Gudel, M. Fortsch, and H.-B. Burgi, Inorg. Chem., 33, 545 (1994)). It is reported that, when used as a dopant in a EL device, this material affords an internal quantum efficiency of greater than 85%. This high efficiency can result from collecting energy from both the singlet and triplet excited states of the host (3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) was the host material). Highly efficient red electrophosphorescence has also been reported, using bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3)iridium(acetylacetonate) [Btp2Ir(acac)] as the dopant (Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App. Phys. Lett., 78, 1622-1624 (2001)).
Small-molecule, vacuum-deposited OLEDs having high efficiency have also been demonstrated with Ir(ppy)3 as the dopant and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)). The layer containing the dopant and host was deposited adjacent to a hole-blocking layer, 2,9-dimethyl-4,7-diphenyl-phenanthroline (bathocuproine, BCP). Other device structures for generating electrophosphorescence have also been documented, for example see C. Adachi, M. A. Baldo, S. R. Forrest, and M. E. Thompson, Appl. Phys. Lett., 77, 904 (2000).
The emission wavelengths of cyclometallated Ir(III) complexes of the type IrL3 and IrL2L′, such as the green-emitting fac-tris(2-phenylpyridinato-N,C2′)Iridium(III) and bis(2-phenylpyridinato-N,C2′)Iridium(III)(acetylacetonate) can be shifted by substitution of electron donating or withdrawing groups at appropriate positions on the cyclometallating ligand L, or by choice of different heterocycles for the cyclometallating ligand L. The emission wavelengths can also be shifted by choice of the ancillary ligand L′. Examples of red emitters are the bis(2-(2′-benzothienyl)pyridinato-N,C3′)Iridium(III)(acetylacetonate) and tris(2-phenylisoquinolinato-N,C8′)Iridium(III). A blue-emitting example is bis(2-(4′,6′-diflourophenyl)-pyridinato-N,C2′)Iridium(III)(picolinate).
Other important phosphorescent dopants include cyclometallated Pt(II) complexes such as cis-bis(2-phenylpyridinato-N,C2′)platinum(II), cis-bis(2-(2′-thienyl)pyridinato-N,C3′)platinum(II), cis-bis(2-(2′-thienyl)quinolinato-N,C5′)platinum(II), or (2-(4,6-diflourophenyl)pyridinato-N,C2′)platinum (II) acetylacetonate. Pt(II) porphyrin complexes such as 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum(II) are also useful phosphorescent dopants.
Without being limited to a particular theory, phosphorescence can be found among transition metal ion octahedral complexes having d6 electron configuration and square planar complexes having d8 electron configuration, each also having as the lowest-energy excited state a metal-to-ligand charge transfer transition or a ligand pi-pi* transition that is capable of mixing with a nearby metal-to-ligand charge transfer. Suitable transition metal ions for phosphorescent complexes include preferably the second or third transition series, or more preferably the third transition series (Hf, Nb, W, Re, Os, Ir, Pt, Au).
Other examples of phosphorescent compounds include compounds having interactions between atoms having d10 electron configuration, such as Au2(dppm)Cl2 (dppm=bis(diphenylphosphino)methane) (Y. Ma et al, Appl Phys. Lett., 74, 1361 (1998)). Still other examples of phosphorescent dopants include coordination complexes of the trivalent lanthanides such as Tb3− and Eu3+ (J. Kido et al, Appl. Phys. Lett., 65, 2124 (1994)). While these latter dopants do not have triplets as the lowest excited states, their optical transitions do involve a change in spin state of 1 and thereby can harvest the triplet excitons in OLED devices.
Phosphorescent dopants are also described in WO 00/57676, WO 00/70655, WO 01/41512 A1, WO 02/15645 A1, WO 01/93642 A1, WO 01/39234 A2, WO 02/071813 A1, EP 1 239 526 A2, EP 1 238 981 A2, EP 1 244 155 A2, WO 02/074015 A2, JP 2003 073387A, JP 2003 073388A, JP 2003 059667A, JP 2003 073665A, U.S. Pat. Nos. 6,458,475 B1; 6,451,455 B1; 6,239,085 B1; 6,413,656 B1; 6,515,298 B2; 6,451,415 B1; 6,097,147; 6,573,651 B2; U.S. Patent Application Publication Nos. 2002/0197511 A1; 2003/0124381 A1; 2003/0072964 A1; 2003/0068528 A1; 2003/0059646 A1; 2003/0054198 A1; 2003/0017361 A1; 2002/0100906 A1; 2003/0068526 A1; 2003/0068535 A1; 2003/0141809 A1; 2003/0040627 A1; 2002/0121638 A1, and 2002/0121638 A1.
In a similar fashion, high efficiency polymer OLEDs containing phosphorescent dopants, such as Ir(ppy)3, have also been reported, for example, see M.-J. Yang and T. Tsutsui, Jpn. J. Appl. Phys., 39, L828 (2000).
Suitable host molecules for phosphorescent dopants must have the energy of their triplet states about equal to or above that of the phosphorescent dopant in order that the triplet exciton can be transferred efficiently from the host to the phosphorescent dopant. However, the band gap of the host should not be chosen so large as to cause an unacceptable increase in the drive voltage of the OLED. Suitable host types are described in WO 00/70655 A2; WO 01/39234 A2; WO 01/93642 A1; WO 02/074015 A2; and WO 02/15645 A1. Suitable hosts include certain aryl amines and carbazole compounds.
In addition to suitable hosts, an OLED device employing a phosphorescent dopant often requires at least one exciton- or hole-blocking layer to help confine the excitons or electron-hole recombination centers to the light-emitting layer including the host and dopant. In one embodiment, such a blocking layer would be placed between the electron-transporting layer and the light-emitting layer. In this case, the ionization potential of the blocking layer should be such that there is an energy barrier for hole migration from the host into the electron-transporting layer, while the electron affinity should be such that electrons pass more readily from the electron-transporting layer into the light-emitting layer including host and dopant. It is further desired, but not absolutely required, that the triplet energy of the blocking material be greater than that of the phosphorescent dopant. Suitable hole-blocking materials are described in WO 00/70655 A2 and WO 01/93642 A1. Two examples of useful materials are bathocuproine (BCP) and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)Aluminum(III) (BAlQ).
Notwithstanding these developments, there remains a need for new organometallic materials that will function as phosphorescent emitters, especially in the blue-emitting region. To date, most of the phosphorescent complexes developed for OLED applications have emission spectra with peaks in the red and green region. U.S. Patent Application Publications Nos. 2002/0134984A1 and 2004/0068132A1 also disclose iridium complexes that emit in the blue region, even though the colors are not saturated. Recently, B. W. D'Andrade, J.-Y Tsai, C. Lin, M. S. Weaver, P. B. Mackenzie, J. Brown, “Efficient White Phosphorescent Organic Light-Emitting Devices,” SID Symposium Digest, Vol. 38, pp. 1026-1029, 2007 have reported a sky-blue phosphorescent OLED with an operational lifetime of 15,000 hr. at an initial brightness of 200 cd/m2 and 9.5% external quantum efficiency. However, with CIE coordinates of (0.16, 0.37), this material is not a sufficiently saturated blue for commercial full-color applications. Research on higher triplet-energy emitters has been reported. These emitters are often iridium complexes that have two cyclometalated (C^N) ligands and one bidentate ancillary ligand (N^N) (J. Li, P. I. Djurovich, B. D. Alleyne, M. Yousufuddin, N. N. Ho, J. C. Thomas, J. C. Peters, R. Bau, M. E. Thompson, “Synthetic Control of Excited-State Properties in Cyclometalated Ir(III) Complexes Using Ancillary Ligands,” Inorg. Chem., Vol. 44, pp. 1713-1727, 2005). The (N^N) ancillary ligands are believed to render these blue phosphors unstable in devices. To date, the design and synthesis of efficient “deep blue” phosphorescent emitters remains one of the most challenging tasks in the OLED community.
Therefore, there remains a need for new efficient phosphorescent materials, particularly materials that produce their emission in the blue region of the visible spectrum. It is a problem to be solved to provide new blue phosphorescent emitting materials that can function under practical operating conditions in an OLED device. Desirably, such a material can be developed to improve on device efficiency and operational stability.